Speckle is a serious problem associated with laser display systems. Speckle noise is an interference pattern resulting from reflection or transmission of highly coherent (both spatial and temporal) light from optically rough surfaces (RMS surface height deviations on the same order or scale as the wavelength of the light beams) such as display screens. Speckle noise can be reduced by superimposing a number of uncorrelated speckle patterns, obtained by diversity of time, space, frequency or polarization of light as taught by Goodman.
Past efforts of reducing speckle noise are numerous. Many of the conventional speckle reduction approaches are based on disturbance of temporal coherence of laser beams such as wavelength (frequency) diversity achieved from multiple lasers associated with beam combiner, broadband lasers, or pulse lasers especially short-pulse lasers, in the order of picoseconds generated by, e.g., mode-locking techniques.
Broadening the linewidth of a laser light to reduce the temporal coherence can be achieved by a number of ways. For example, in U.S. Pat. No. 5,274,494, Rafanelli, et al. described a system, in which a coherent light impinged into a Raman cell and the spectral bandwidth was broadened. In U.S. Pat. Nos. 6,600,590 and 6,625,381, Roddy and Markis invented a method of using RF signal injection for speckle reduction. However, the success was limited because only a small number of longitudinal modes could be produced. Ideally, the spectral bandwidth for a projection display light source should be on the order of several nanometers, e.g., 5- to 15-nm. Such a light source could be considered quasi-monochromatic, which is sufficiently broadband for the cancellation of speckle yet sufficiently narrowband for color purity. Unfortunately, there is simply no laser-based RGB light source in existence with these properties.
In U.S. Pat. No. 6,191,887, Michaloski, et al. disclosed a speckle reduction system, which divides pulses of coherent radiation into successions of temporally separated and spatially aberrated pulselets. These pulselets, which are circulated through delay lines and modified by spatial aberrators therein, produce a succession of different speckle patterns that can be averaged within the integration interval of a detector to reduce speckle contrast.
Independent, i.e., uncorrelated and non-interfering, speckle patterns can also be created due to disturbance of the phase coherency between the interfering beams and speckle noise is reduced by time averaging of these patterns. One approach to speckle reduction by time averaging of the phase shift was described in U.S. Pat. No. 4,035,068, wherein a rotating diffuser was used.
Another approach to reducing the temporal coherence is to split the illuminating wavefront into beamlets and delay them relative to each other by an interval longer than the coherence time of the laser, as taught by Rasmussen et al. in U.S. Pat. No. 5,224,200, as well as by Wang et al.
Another family of speckle reduction solutions is based on disturbance of spatial coherence of the laser beams through, e.g., optical path randomization. This can be realized by vibrating the display screen to dynamically vary the speckle pattern, as described by Thompson et al. in U.S. Pat. No. 5,272,473, or by coupling the laser light into a multimode optical fiber and vibrating the fiber to cause mode-scrambling, as described in U.S. Pat. No. 3,588,217, issued to Mathisen. These approaches, however, may not be suitable for high-speed machine vision systems.
Alternatively, reducing the visibility of the speckle pattern can be based on a diffusing element that moves or vibrates within the projector system, typically at an intermediate image plane, for angle diversity, as disclosed in U.S. Pat. No. 4,035,068, issued to Rawson. One limitation of such approaches is that the diffusion must occur precisely at the image plane. In addition, complicated projection lenses are required to provide an intermediate image plane. A more preferable approach involves dynamically diffusing the laser beam in the illumination path through a rotating diffuser or a rotating plate of variable thickness, as disclosed in U.S. Pat. Nos. 3,490,827, 5,313,479, and 6,005,722. However, it is hard to control the illumination brightness while achieving sufficient uniformity in a compact system.
Reduction of spatial coherence can also be realized by the use of a rotating microlens array to create a plurality of incoherent light sources, as described in U.S. Pat. No. 6,081,381.
In addition to temporal and spatial averages, speckle noise may be reduced by other means. For example, speckle reduction may be based on polarization diversity, in particular, based on overlapping two beams with two orthogonal polarizations, as disclosed in U.S. Pat. No. 4,511,220. Another example of using polarization averaging to reduce laser speckle is given in U.S. Pat. No. 6,956,878. These methods, however, may be insufficient for highly coherent lasers because there are only two uncorrelated components. The performance may be improved by increasing the number of polarization states of the laser beam, as described by Miron in United States Patent Application No. 20050008290.
Another non-time averaging approach for reducing the speckle was based on diversity of phase retardations. For example, as described in U.S. Pat. No. 6,169,634, a plurality of optical fibers of various lengths introduced different phase retardations of the incident wavefront. Phase retardation may also be a result of two polarization components traveling in an anisotropic optical element. Again, the speckle reduction based on these schemes was insignificant.
Attempts to speckle reduction in laser display systems continue in recent years. As an example, in U.S. Pat. No. 6,323,984, Trisnadi described a laser projection system, which used a wavefront modulator to change the spherical wavefront incident on it. The wavefront modulator is located at an intermediate image plane within the imaging system, rather than within the illumination system. As a result, the wavefront at the output is no longer spherical but is still spatially coherent with well-defined phase relationships between the different points of the wavefront. By vibrating the wavefront across a direction perpendicular to the incident beam, laser speckle can be reduced by time averaging. However, the effects are limited.
More recently, in U.S. Pat. No. 6,747,781 and United States Patent Application No. 20040008399, Trisnadi described a method of reducing speckle, which includes steps of dividing a laser illuminated area into phase cells, subdividing the phase cells into cell partitions, and applying a temporal phase variation to the cell partitions within an integration time of an intensity detector viewing the laser illuminated area.
As another example, in U.S. Pat. No. 6,577,429, Kurtz, et al. demonstrated a laser display system, in which speckle was reduced by employing an electrically controllable de-speckle modulator positioned within the illumination portion of the optical system rather than within the imaging optics. One drawback of such image display systems is that they rely upon relatively large and massive moving components, which makes it difficult to achieve real-time projection of high-resolution motion images.
Furthermore, in U.S. Pat. Nos. 6,304,237 and 6,774,881, as well as in United States Patent Application No. 20020018036, issued to Karakawa, an RGB pulsed laser source was demonstrated. Speckle noise in the green spectral range was reduced by introducing spatial incoherency via an etalon and in the red spectral range was reduced by multimode operation and spectrum incoherency. Unfortunately, speckle reduction in the blue spectral range was ineffective. In addition, the laser operation was limited to pulsed mode, which generally increases amplitude noise, and the speckle reduction was limited.
Similarly, in U.S. Pat. No. 6,483,556, Karakawa demonstrated another laser video display system, comprising a red light generated from KTA based intracavity optical parametric oscillation (OPO) and SFM, a green light from a CW lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser, and a blue light from frequency doubled Ti:Sapphire laser. Spectral noise of the green light was reduced with controlled multimode operation in transverse direction. However, no speckle reduction in the red and blue spectral ranges was mentioned. Again, the operation was restricted to pulsed mode.
A bandwidth-enhanced laser display system was invented by Manni, et al. As disclosed in U.S. Pat. No. 6,975,294, the laser image system includes 1-D or 2-D arrays of independent laser emitters. Each emitter has a spectral bandwidth, centered at some arbitrary wavelength, which can be slightly different from the central wavelengths of other elements in the array. Spatial superposition of these radiations results in a broadened bandwidth, which reduces speckle in a displayed image.
Previous methods for speckle reduction such as oscillatory motion, active diffuser, diffractive optical elements with or without rotation, wavefront modulation, spatial light modulation based on electromechanical grating, arrays of independent laser emitters, multiple Raman cells, or Doppler shift are complex and cause significant power losses. In addition, these methods may be ineffective or merely partially effective to some wavelengths that cannot be directly generated from a laser diode or from harmonic generation of a diode-pumped solid-state (DPSS) laser or from an ultra-compact laser based on the SFM schemes.